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Molecular and Cellular Biology, October 2000, p. 7214-7219, Vol. 20, No. 19
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Degradation Signals Recognized by the Ubc6p-Ubc7p Ubiquitin-Conjugating Enzyme Pair

Tamar Gilon, Orna Chomsky, and Richard G. Kulka*

Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Received 7 February 2000/Returned for modification 7 April 2000/Accepted 17 July 2000

Proteolysis by the ubiquitin-proteasome system is highly selective. Specificity is achieved by the cooperation of diverse ubiquitin-conjugating enzymes (Ubcs or E2s) with a variety of ubiquitin ligases (E3s) and other ancillary factors. These recognize degradation signals characteristic of their target proteins. In a previous investigation, we identified signals directing the degradation of beta -galactosidase and Ura3p fusion proteins via a subsidiary pathway of the ubiquitin-proteasome system involving Ubc6p and Ubc7p. This pathway has recently been shown to be essential for the degradation of misfolded and regulated proteins in the endoplasmic reticulum (ER) lumen and membrane, which are transported to the cytoplasm via the Sec61p translocon. Mutant backgrounds which prevent retrograde transport of ER proteins (hrd1/der3Delta and sec61-2) did not inhibit the degradation of the beta -galactosidase and Ura3p fusions carrying Ubc6p/Ubc7p pathway signals. We therefore conclude that the ubiquitination of these fusion proteins takes place on the cytosolic face of the ER without prior transfer to the ER lumen. The contributions of different sequence elements to a 16-amino-acid-residue Ubc6p-Ubc7p-specific signal were analyzed by mutation. A patch of bulky hydrophobic residues was an essential element. In addition, positively charged residues were found to be essential. Unexpectedly, certain substitutions of bulky hydrophobic or positively charged residues with alanine created novel degradation signals, channeling the degradation of fusion proteins to an unidentified proteasomal pathway not involving Ubc6p and Ubc7p.

* Corresponding author. Mailing address: Department of Biological Chemistry, Alexander Silberman, Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel. Phone: 972 2 6585408/7. Fax: 972 2 6585811. E-mail: dick{at}leonardo.ls.huji.ac.il.

Molecular and Cellular Biology, October 2000, p. 7214-7219, Vol. 20, No. 19
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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